L1/L2 GPS receiver

ABSTRACT

In a system and method for simultaneously receiving or switching between dual frequency carrier signals in a GPS receiver, the GPS receiver is adapted to utilize different harmonics of a sub-harmonic frequency generator, which may include a lower frequency voltage controlled oscillator (VCO) to detect the L1 and L2 GPS carriers. A sub-harmonic mixer may be used to simultaneously down convert the L1 and L2 signals to a lower intermediate frequency (IF). A second mixer may be an image reject (IR) mixer used to separate the downconverted L1 and L2 signals. This mixer may be configured to simultaneously monitor the L1 and L2 signals, or to switch between the L1 and L2 signals. High frequency switching is not required of the radio frequency (RF) input or local oscillator signals, and simultaneous L1 and L2 reception is enabled without and 3 dB image noise degradation. This system and method minimizes the RF components and power dissipation in a dual frequency GPS receiver, while optimizing the functionality and performance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/715,656 filed on Nov. 18, 2003 now U.S. Pat. No. 7,035,613,which is a continuation of U.S. patent application Ser. No. 09/732,956filed on Dec. 7, 2000, now U.S. Patent No. 6,675,003, incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to radio frequency receivers, and morespecifically to multiple band global positioning system (GPS) receiversused for navigation.

2. Description of the Related Art

GPS satellites transmit data at two radio frequency (RF) system carrierfrequencies: 1575.42 MHz (L1) and 1227.6 MHz (L2). GPS data from bothcarriers can be used to increase the position accuracy, and to providecarrier selectivity in case of interference or jamming of one of thecarriers.

A GPS receiver designed to receive the L1 and/or L2 carriers requires amethod for receiving both signals simultaneously or efficientlyswitching between the signals. One solution is to duplicate all receiverparts and functions for the L1 and L2 bands. However, for low-powerportable receivers, it is desirable to integrate the L1 and L2 functionsas much as possible, to minimize the number of RF functions and powerdissipation.

It has been known for L1/L2 receivers to use parallel RF paths and/or RFswitching of the input and/or local oscillator (LO) signals. Forexample, U.S. Pat. No. 5,883,597 discloses an L1/L2 GPS receiver inwhich the LO is switched between three frequencies to select “L1 only,”“L2 only” or “L1 and L2.” However, this requires the LO to be tunableover a very wide frequency range of about 696 MHz, from approximately1054 MHz to 1750 MHz, which makes on-chip integration difficult.Further, due to practical design limitations, this may require switchingbetween two or three tuned oscillators, which may result in excessivepower consumption for multiple voltage controlled oscillators (VCOs).Also, in the “L1 and L2” mode, this receiver may suffer a 3 dB noisepenalty due to image noise. Switching of the LO signal may also requireresynchronization of tracking loops, which reduces receiver responsetime for time sensitive applications.

U.S. Pat. No. 5,678,169, for example, discloses an L1/L2 receiver inwhich the VCO and LO frequency is fixed exactly halfway between the L1and L2 carriers, as in the “L1 and L2” mode of the above referred toreceiver. This receiver uses switched L1 and L2 filters which eliminatethe problem of the 3 dB image noise. However, this receiver may not becapable of true simultaneous L1 and L2 detection, since the L1/L2selection is done by RF switches before the mixer.

U.S. Pat. Nos. 5,040,240 and 5,736,961, for example, disclose L1/L2receivers which use parallel RF paths for the downconversion. U.S. Pat.No. 5,040,240 uses a common VCO with a series of different dividers andmultipliers for the L1 and L2 downconversions. However, due to theduplication of RF functions, these methods are not optimum for highintegration and low-power.

Therefore, those concerned with the development and use of improved dualfrequency carrier signal receiver systems and methods have recognizedthe need for improved systems and methods for enabling simultaneous dualfrequency capabilities without requiring radio frequency switches orlocal oscillator switching.

Accordingly, the present invention fulfills these needs by providingefficient and effective systems and methods for simultaneously receivingor switching between dual frequency carrier signals in a highlyintegrated, low power receiver.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides a systemand method for simultaneously receiving or switching between dualfrequency carrier signals.

By way of example, and not by way of limitation, the present inventionprovides a new and improved system for simultaneously receiving orswitching dual frequency carrier signals, without local oscillatorswitching or radio frequency switches.

More particularly, the present invention includes a sub-harmonicfrequency generator, which may include a sub-harmonic VCO, withdifferent harmonics of the sub-harmonic frequency VCO providing thelocal oscillator signals for the L1 and L2 carriers. Downconversion inthe sub-harmonic frequency generator or a first mixer then produces twointermediate frequencies (IF) for the L1 and L2 carriers. The VCOfrequency and harmonic orders may be chosen such that the differencebetween these two IF signals is twice the desired final IF. The final IFmay be obtained through a second mix in a second mixer with an LO signalthat is halfway between the L1 and L2 IF frequencies. Since these IFsignals generated in the first mixer are on either side of the LOfrequency they can be separated by having the second mixer be an imagereject mixer. The image reject mixer can be used to receive L1 and L2simultaneously using both its outputs, or to switch between L1 and L2.The selection is accomplished by interchanging the ‘I’ and “Q” LO inputsignals of the second IR mixer. Since this switching is done at a lowerIF frequency it does not cause unlocking of the phase locked loop (PLL)or the receiver tracking loop.

This receiver architecture is chosen to minimize power dissipation,while optimizing integration and performance. Operation of an on-chipintegrated VCO at a frequency three to four times lower than the L1/L2RF carriers saves power in the VCO and PLL. Switching at the IFfrequency consumes less power compared to RF or LO switching, and doesnot degrade the receiver noise figure. RF switches introduce front-endloss which degrades the receiver noise figure. Only one external splitband filter is required at the front end to reject the first imagefrequencies for the L1 and L2 downconversion. The second image isrejected by the image reject function of the second mixer. There is no 3dB degradation for simultaneous L1/L2 herein.

A single fixed frequency VCO eliminates the need of LO switching, andeliminates the need of RF switches, while still providing simultaneousL1 and L2 capability.

Although the preferred embodiment described is an L1/L2 GPS receiver,the systems and methods described herein can be used for any dualfrequency RF receiver.

The above and other objects and advantages of the invention will becomeapparent from the following more detailed description, when taken inconjunction with the accompanying drawing of an illustrative embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a circuit diagram of a dual frequency carrier signalreceiver, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to an improved system and method forsimultaneously receiving or switching dual frequency carrier signals.The improved system and method provides efficient and effectivesimultaneous receiving or switching, without radio frequency switches orlocal oscillator switching. The preferred embodiments of the improvedsystem and method are illustrated and described herein by way of exampleonly and not by way of limitation.

Referring now to the FIGURE, which illustrates a system 10 forsimultaneously receiving or switching between dual frequency carriersignals, comprising a receiver 12 which is preferably a GPS receiver forthe L1 and L2 carriers. The front-end of the receiver 12 consists of adual band antenna 14 for receiving the dual frequency carrier signals,and a wide band low noise amplifier (LNA) 16, with 1.2 GHz to 1.6 GHzbandwidth, for amplifying the L1 and L2 carriers. A split band surfaceacoustic wave (SAW) filter 18 is then used to pass the L1 and L2 bandsand reject other frequencies. The out-of-band rejection of this filter18 is adapted to be high enough to sufficiently attenuate the firstimage frequencies, as set forth below.

The system 10 further includes a sub-harmonic frequency generator, forgenerating a sub-harmonic frequency so as to enable harmonics of thesub-harmonic frequency to generate local oscillator frequency signalsfor the dual frequency carrier signals, and for mixing the dualfrequency carrier signals with the local oscillator frequency signals,to generate distinct intermediate frequency signals for each dualfrequency carrier signal. The sub-harmonic frequency generator maycomprise a voltage controlled oscillator (VCO) 20, for generating thesub-harmonic frequency, and a first mixer 22, which may comprise asub-sampling mixer for mixing the dual frequency carrier signals withthe local oscillator frequency signals. The sub-harmonic frequencygenerator alternatively may comprise a sub-harmonic mixer, forgenerating the sub-harmonic frequency, and for mixing the dual frequencycarrier signals with the local oscillator frequency signals, the choiceof which as an alternative to the sub-sampling mixer may depend on thefrequency plan. The sub-sampling mixer 22 may be adapted to includeswitches comprising N-channel metal oxide semiconductor transistors. Thesignals from the first mixer 22 are input into a second mixer 24.

The VCO 20 comprises a sub-harmonic voltage controlled oscillator,adapted to generate a sub-harmonic frequency thereof and to enableharmonics of the sub-harmonic frequency to generate LO frequency signalsfor the dual frequency carrier signals. The signal from the VCO 20 isinput into the first mixer 22, and is input into a divide by threedivider 26 and a divide by five divider 28, from which the I and Qphases are input into an IQ select switch 30. The IQ select switch 30selectively switches between the I and Q phases, and the I and Q phasesare input into the second mixer 24. The IQ select switch 30 enablesefficient selection and switching to be accomplished between the L1 andL2 signals by enabling the interchanging of the I and Q LO input signalsin the second mixer 24. The second mixer 24 is an image reject (R)mixer, which is adapted to separately receive the L1 and L2 signals, andincludes a pair of outputs. It can be configured to simultaneouslyprovide both L1 and L2 signals using both mixer outputs and dual outputpaths, or to switch between the L1 and L2 mixer outputs using only oneoutput path. The selection in the second mixer 24 between the L1 and L2signals is preferably implemented by interchanging the “I” and “Q”signals of the LO frequency signals.

The IF signals generated in the first mixer 22 are preferably on eitherside of an LO frequency signal, and are adapted to be separated by thesecond mixer 24. The second mixer 24 is further adapted to generate thefinal IF upon mixing with an LO frequency signal which is intermediateto the L1 and L2 IFs. The LO frequency adapted to be mixed with the L1and L2 IFs in the second mixer 24 is approximately halfway between theL1 and L2 IFs. The final IF signal is input into a low-pass filter 32and automatic gain control (AGC) amplifier 34 before being sampled by ananalog to digital (A/D) converter 36. The frequency and the harmonics ofthe VCO 20 are preferably chosen such that the difference between thefirst IFs is approximately twice the desired final IF.

Several constraints influence the choice of a frequency plan for thereceiver 12 and the frequency of the VCO 20. For low power dissipationin the VCO 20 and a PLL 38 it is desirable to have the VCO frequency assmall as possible. This may increase the sub-harmonic ratio, which isthe number of times the VCO signal must be multiplied in the mixer 22before mixing with the carrier. The preferred frequency of the VCO 20 isabout 401.62 MHz. The noise figure of the mixer 22 may increase withincreasing sub-harmonic ratios, which may degrade receiver performance,and require more RF gain. A sub-harmonic ratio of 3 to 4 is preferred,lowering the VCO power significantly while minimizing the mixer noise.Another constraint is that the L1 and L2 IF signals after the firstmixer 22 should be high enough to enable sufficient rejection of theimage frequencies by the RF SAW filter 18.

If n and m are the sub-harmonic ratios for L1 and L2 respectively, andf_(vco) is the VCO frequency, then the first IF frequencies for the L1and L2 carriers are given by f_(IF1,L1)=|nf_(VCO)−1227.6 MHz|, and byf_(IF1,L2)=|mf_(VCO)−1575.42 MHz. The LO frequency for the second mixer24 is given by f_(L02)≅(f_(IF1,L1)+f_(IF1,L2))/2, and the finalintermediate frequencies are given by f_(IF2)≅|f_(IF1,L1)−f_(IF1,L2)|/2.

Preferably n=3 and m=4 for the sub-harmonic ratios. Using convenientinteger dividers to generate the 2^(nd) LO and sampling frequency, theoptimum frequency plan is then given by: f_(VCO)=401.63 MHZ,f_(IF1,L1)=31.10 MHz, f_(IF1,L2)=22.71 MHZ, f_(L02)=f_(VCO)/15=26.78MHz, f_(IF2,L1)=4.32 MHz, f_(IF2,L2)=4.07 MHz and sampling frequencyf_(S)=fvco/24=16.73 MHz. Oversampling by a factor of approximately foureliminates any degradation due to noise folding, and provide samplesthat are close to 90 degree I, Q samples which minimize processing loss.The first image frequencies for mixer 22 are 62.2 MHz and 45.4 MHz awayfrom the L1 and L2 carriers respectively. This does not put excessivedemands on the split band RF SAW filter 18, which would require a 25 MHzto 30 MHz 3-dB bandwidth for each band.

The sub-harmonic mixer 22 should have sufficient conversion gain andsufficiently low noise figure for the n=3 and m=4 sub-harmonic mixingproducts, to minimize impact on the receiver sensitivity. A sub-samplingintegrated switched capacitor implementation of the mixer 22 ispreferred for optimum performance at both the 3^(rd) and 4^(th)harmonics.

The first mixer 22 is preferably adapted to mix the third harmonic ofthe VCO 20 with the L2 carrier, and the fourth harmonic of the VCO 20with the L1 carrier.

In accordance with the present invention, true simultaneous L1/L2reception capability is provided, with the flexibility to choosesimultaneous or switched operation, and without 3 dB degradation innoise figure.

In the present invention, an efficient means of switching between L1 andL2 at the IF frequency is provided by exchanging the I and Q LO signalsof the 2^(nd) IR mixer. Lower frequency switching at the IF dissipatesless power than the RF and LO switching techniques of prior switching,and also does not disturb the phase lock of the PLL and tracking loops.IF switching in the system after amplifier gain therein also eliminatesthe noise figure degradation caused by front-end RF switches.

Pursuant to the invention, the sub-harmonic VCO for a dual frequency GPSreceiver operating at one fourth the L1 frequency is adapted to savepower dissipation in the VCO and PLL.

In accordance with the invention, on-chip RF functions and externalcomponents required for a high functionality GPS L1/L2 receiver areminimized. An external high-Q IF filter is not required, and the VCO canbe easily be integrated on the chip.

It will be apparent from the foregoing that, while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

1. A system for receiving or switching between dual-frequency carriersignals, comprising: a local oscillator generating a signal of aselected frequency; and an image reject mixer that mixes thedual-frequency carrier signals simultaneously with the generated signalfrom the local oscillator to separate the dual frequency carrier signalsat an output terminal of the image reject mixer, and switches betweenthe dual-frequency carrier signals, responsive to exchanging the I and Qphases of the signal of the local oscillator.
 2. A system for receivingor switching between dual-frequency carrier signals, comprising: asubharmonic frequency generator, which generates a sub-harmonicfrequency for use in generating local oscillator frequency signals forthe dual-frequency carrier signals; and a single mixer which mixes thedual-frequency carrier signals with the local oscillator frequencysignals to simultaneously generate distinct intermediate frequencysignals for each dual-frequency carrier signal at an output terminal ofthe single mixer.
 3. A system as in claim 2, wherein the localoscillator frequency signals are adapted to include I and Q phases.
 4. Asystem as in claim 3, further comprises an image reject mixer, whichreceives simultaneously and separates the dual-frequency carriersignals, and switches between the dual frequency carrier signals.
 5. Asystem as in claim 4, wherein the image reject mixer is responsive toexchanging the I and Q phases of the local oscillator frequency signals.